1. Field of the Invention
The present invention relates to an electronic circuit for driving a laser diode.
2. Related Background Art
There are known laser driver circuits of a peak detection type for stabilizing both the average output power and the extinction ratio of a laser diode.
Japanese Patent Application Laid-Open No. 4-13977 discloses an AC-coupled laser driver circuit. The driver circuit disclosed in the application is configured to detect an average output current from a monitor photodiode and to control the magnitude of a bias current supplied to a laser diode depending on the detected average current. This driver circuit controls the modulation current based on the average and peaks of the optical output from the laser diode. Controlling the bias current stabilizes the average output power, and controlling the modulation current stabilizes the extinction ratio.
Japanese Patent Application Laid-Open No. 9-92916 discloses another laser driver circuit. This driver circuit is configured to detect the average and peaks of an output current from a monitor photodiode to control the duty ratio of a driving pulse depending on the detected average current, and to control the magnitude of the current according to the peaks. Controlling the duty ratio stabilizes the average output power, and controlling the magnitude of the current stabilizes the extinction ratio.
For detecting the amplitude of the output current from the photodiode, it is common practice to use a current-voltage converter having a broad bandwidth to convert the output current to a voltage signal (i.e., monitoring signal) and detect the peak-to-peak value thereof. However, the laser diode driving current (up to several tens mA induces crosstalk noise in the photodiode output current (several tens μA to several, therefore, it is difficult to accurately detect the peaks of the monitoring signal. Consequently, the optical output from the laser diode is likely to be unstable.
An object of the present invention is to provide a laser driver circuit capable of adequately stabilizing the optical output from a laser diode.